The present invention relates to an implant material (a material to be buried) for a living body hard tissue and a process for producing the same. More particularly, the invention relates to such an implant material for a living body hard tissue having less living body rejection, a proper affinity to the living body tissue, rigid coupling to tissue and excellent durability in vivo as materials of an artificial bone or tooth to fill or replace the defective portion of the bone or tooth and a process for producing the same.
Materials buried in vivo as artificial bone or tooth have heretofore been discussed and tried over a long period of years, but there are still a number of problems. Material such as ceramics, glass or carbon have been recently developed, instead of the conventional metal or high molecular resin material, and put to practical use.
The required properties of the implant material variously depend upon the shape, dimensions and function of the material at the implant position, and designs and selection of materials which have the required properties also depend largely upon the very difficult conditions present in the living materials.
The conditions of the implant material necessary to safely achieve the function in vivo are as follows:
(1) The implant material has no toxicity, no tissue stimulation, no carcinogenesis, no allergy, nor defects such as destructive action or the like of tissue in the vicinity.
(2) The material should not induce an immune reaction (which is an action of discharging the body out of the living body).
(3) The material should not be absorbed (decomposed or erased) in vivo.
(4) The material should not be ionized or dissolved by the influence of blood, lymph or the like.
(5) The material should have suitable affinity to the live body tissue and be bonded to it.
(6) The material should have high mechanical strength such as strength against compression, high durability against stress in use and resistance to deterioration in strength under a living body environment.
(7) The material should be readily handled and subjected to an easy sterilization.
(8) The material should be able to be adjusted in hardness and elasticity in a degree equal to or slightly larger than the hard tissue of the living body and to have a natural state as near as possible even if the material is integrated with the tissue.
When the materials are evaluated in comparison with the above-described requirements, the metal materials have problems in the conditions listed in the above paragraphs (1), (2), (4) and (8), and the plastic materials frequently have problems in the above paragraphs (2), (3) and (6). On the other hand, the ceramic materials which have been recently noted almost meet the conditions peculiar for the living body tissue in the above paragraphs (1) to (5). The reasons why the ceramic materials were not heretofore used were that they did not have sufficient mechanical strength listed in the above paragraph (6) and that the materials were disadvantageously deteriorated in strength under the environment in vivo with much water content. In this respect, the above drawbacks can be improved due to the developments of artificial sapphire single crystal, polycrystalline alumina material or sintered apatite hydroxide, which are partly employed in a practical use. However, the alumina material of the former has remarkably high hardness and elastic modulus, which causes problems such as an induction of defects in which a stress applied to the living body is concentrated at the buried material, unreasonable stress is acted in the environmental tissue to destroy the tissue and in an extreme case, is arrived at the tissue of a nerve, thereby damaging the nerve. Thus, the alumina material still remains insufficient under the requirement listed in the above paragraph (8). The sintered synthesized apatite hydroxide of the latter is a material which is extremely near the hard tissue of the living body and has less problems than in the case of the alumina material, but has insufficient mechanical strength, and is encountered in difficulties and complications in the synthesis, molding and sintering techniques of the apatite powder. Further, the following means are employed when repairing the position of broken bone in the normal living body hard tissue (bone and tooth) in operation, burying an artificial joint, and implanting an artificial tooth root so as to secure an implant material to the body:
(1) A method of self-locking the implant material to a bone tissue by devising in a structure or a shape.
(2) A method of mechanically securing by using screws and bolts, nuts.
(3) A method of bonding the implant material to the bone with an adhesive such as a medical cement. However, the implant material is slackened by any of the above methods for a long period of time, and even if the implant material itself does not have any defect, it is necessary in some cases to consider in case of exfoliating and replacing the implant material.
An implant material is buried and implanted as a dental root into an alveolar bone, and a dental crown is secured to the material in a dental treatment, in which case the above-described various hard implant material is used. This material has various shapes such as a natural dental root shape, a pin shape, a blade shape or a screw shape, and any of these materials is used by designing in response to the respective diseases.
In this case, the implant material is used over the tissue in vivo and the surface of the living body. Thus, it is particularly necessary to bond in the boundary between the material and the tissue. More specifically, it is important to bond at the portion which is projected from the living body into the surface of the implant material. If this technique cannot be preferably carried out, unpreferable results are induced due to the contamination of bacterias from the living body or the impregnation of detrimental substances.
In view of the present status that the above-described drawbacks and problems still remain at present, the inventor of the present invention has aimed at the points that carbon has been started to be noted as a material for a living body from the half life of 1960, has excellent antithrombotic properties, has no tissue stimulation, has a high affinity to the living body has, a remarkable stability under various environments in and out of the living body as well as excellent mechanical strength, with the result that a number of examples of success that an application of the carbon to an artificial heart valve has already been issued, and has also been employed in a practical use in ligaments, dental root materials and artificial joints. The inventor has further studied repeatedly the materials and structures in which the bond to the living body hard tissue becomes rigid, devised in designing the shape to provide sufficient strength in maintaining the functions of the living body, and has finally succeeded to create an excellent implant material for a living body hard tissue which could not heretofore be obtained.